Inmates at one of California's most notorious prisons have won a convicts' 'badge of honour' by beating up three infamous prisoners - the serial killers Charles Manson and Juan Corona, and Robert Kennedy's assassin, Sirhan Sirhan.

The men were held in the protective housing unit of Corcoran prison where they are closely monitored as prime targets for violence. But when a door to the exercise yard was left open, three assailants burst in.

Authorities are investigating whether the door was left open deliberately or by accident.

Manson, aged 64, was the leader of a cult that killed actress Sharon Tate and others in Los Angeles in 1969. He was not badly hurt in the attack but had his beloved guitar smashed.

A guard said that Saturday's incident was particularly embarrassing because of the tight security which is meant to surround the notorious trio.

'To attack one of these inmates is a big badge of honour,' he said.

Corcoran has been at the centre of controversy since the FBI began investigating allegations that guards deliberately shot seven inmates to death, staged 'gladiator' fights between prisoners, conducted a mass beating of black prisoners, and instigated a rape.